Anisotropic Chemical Etching of Copper Foil: II. Experimental Studies on Shape Evolution

Wet chemical etching of copper foil (35 μm thick) at the base of the photoresist cavity (100 jxm deep) was investigated experimentally in two geometries (width:depth of 1:1 and 5:1), two acidic cupric chloride solutions, and various flow rates in the range of Peclet number between 107 and 2930. Anisotropic etch profiles were obtained for all conditions investigated. In a 3.5M CuCl₂+ 0.5M HC1 + 0.5M KCl etching solution, results indicated that a salt film precipitated on the entire surface and the etch rate was controlled by the rate of conveetive mass transfer of product removal. The etch rate was highly nonuniform (anisotropic) owing to the presence of local microhydrodynamic flow patterns within the cavity which influenced the local mass-transfer rate. In a 0.5M CuCl₂ + 0.5MHCl + 0.5M KCl etching solution it was concluded, based on the observations of surface roughness, that the film precipitated only in the corners of a 5:1 (width:depth) cavity, whereas the rest of the surface dissolved film-free. Moreover, the dependence of etch rate on fluid velocity was weak. For the same solution and 1:1 cavities, dissolution proceeded under conditions of film precipitation on the entire surface.